Datasheet AD7414, AD7415 (Analog Devices) - 7

Data Sheet AD7414/ AD7415 THEORY OF OPERATION CIRCUIT INFORMATION
Configuration functions consist of The AD7414/AD7415 are standalone digital temperature Switching between normal operation and full power-down sensors. The on-chip temperature sensor allows an accurate Enabling or disabling the SCL and SDA filters measurement of the ambient device temperature to be made. Enabling or disabling the ALERT function The 10-bit analog-to-digital converter converts the temperature Setting the ALERT pin polarity measured into a twos complement format for storage in the temperature register. The ADC is made up of a conventional successive-approximation converter based around a capacitor
VDD VDD VDD SUPPLY
digital-to-analog (DAC). The serial interface is I2C-and SMBus-
2.7V TO 5.5V 10k 10k 10k
compatible. The AD7414/AD7415 require a 2.7 V to 5.5 V
10

F 0.1

F 1k
power supply. The temperature sensor has a working
VDD AS SDA
measurement range of −40°C to +125°C. 
C/

P SCL FUNCTIONAL DESCRIPTION GND ALERT
Temperature measurement is initiated by two methods. The 006
AD7414
first uses an internal clock countdown of 800 ms, and a 02463- conversion is performed. The internal oscillator is the only Figure 6. Typical Connection Diagram circuit that is powered up between conversions, and once it
MEASUREMENT TECHNIQUE
times out, every 800 ms, a wake-up signal is sent to power up the rest of the circuitry. A monostable is activated at the A common method of measuring temperature is to exploit the beginning of the wake-up signal to ensure that sufficient time is negative temperature coefficient of a diode, or the base-emitter given to the power-up process. The monostable typically takes voltage of a transistor, operated at constant current. 4 μs to time out. It then takes typically 25 μs for each conversion Unfortunately, this technique requires calibration to null the to be completed. The new temperature value is loaded into the effect of the absolute value of VBE, which varies from device to temperature value register and ready for reading by the I2C device. The technique used in the AD7414/AD7415 is to interface. measure the change in VBE when the device is operated at two different currents. This is given by A temperature measurement is also initiated every time the one-shot method is used. This method requires the user to  V  KT q  ln N  BE write to the one-shot bit in the configuration register when a where: temperature measurement is needed. Setting the one-shot bit to 1 starts a temperature conversion directly after the write K is Boltzmann’s constant. operation. The track-and-hold goes into hold approximately q is the charge on the electron (1.6 × 10–19 Coulombs). 4 μs (monostable time out) after the STOP condition, and a T is the absolute temperature in Kelvins. conversion is then initiated. Typically 25 μs later, the conversion is complete and the temperature value register is loaded with a N is the ratio of the two currents. new temperature value. The measurement modes are compared with a high tempera- ture limit, stored in an 8-bit read/write register. This is applica-
VDD
ble only to the AD7414, because the AD7415 does not have an ALERT pin and subsequently does not have an overtemperature
I I

N
monitoring function. If the measurement is greater than the high limit, the ALERT pin is activated (if it has already been
V
enabled in the configuration register). There are two ways to
OUT +
deactivate the ALERT pin again: when the alert reset bit in the
TO ADC
configuration register is set to 1 by a write operation, and when the temperature measured is less than the value in the T
V SENSING OUT –
LOW register.
TRANSISTOR
This ALERT pin is compatible with the SMBus SMBALERT
SENSING TRANSISTOR
option. 02463-007 Figure 7. Temperature Measurement Technique Rev. G | Page 7 of 19 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS THEORY OF OPERATION CIRCUIT INFORMATION FUNCTIONAL DESCRIPTION MEASUREMENT TECHNIQUE TEMPERATURE DATA FORMAT INTERNAL REGISTER STRUCTURE ADDRESS POINTER REGISTER CONFIGURATION REGISTER (ADDRESS 0x01) TEMPERATURE VALUE REGISTER (ADDRESS 0x00) AD7414 THIGH REGISTER (ADDRESS 0X02) AD7414 TLOW REGISTER (ADDRESS 0x03) SERIAL INTERFACE SERIAL BUS ADDRESS WRITE MODE Writing to the Address Pointer Register for a Subsequent Read Writing a Single Byte of Data to the Configuration Register, THIGH Register, or TLOW Register READ MODE SMBUS ALERT POWER-ON DEFAULTS OPERATING MODES Mode 1 Mode 2 POWER VS. THROUGHPUT Mode 1 Mode 2 MOUNTING THE AD7414/AD7415 SUPPLY DECOUPLING TEMPERATURE ACCURACY VS. SUPPLY TYPICAL TEMPERATURE ERROR GRAPH OUTLINE DIMENSIONS ORDERING GUIDE